The present invention relates to image scanner technology in general and more specifically to a method for varying the optical sampling rate of an image scanner.
Optical scanners generate data signals representative of an object or document by projecting an image of the object or document onto an optical photosensor array. The data signals may then be digitized and stored for later use. For example, the data signals may be used by a personal computer to produce an image of the scanned object or document on a suitable display device.
Most optical scanners use illumination and optical systems to illuminate the object and focus a small area of the illuminated object, usually referred to as a "scan line," onto the optical photosensor array. The entire object is then scanned by sweeping the illuminated scan line across the entire object, either by moving the object with respect to the illumination and optical assemblies or by moving the illumination and optical assemblies relative to the object.
A typical scanner optical system will include a lens assembly to focus the image of the illuminated scan line onto the surface of the optical photosensor array. Depending on the particular design, the scanner optical system may also include a plurality of mirrors to "fold" the path of the light beam, thus allowing the optical system to be conveniently mounted within a relatively small enclosure. In order to allow a smaller photosensor array to be used, most optical systems also reduce the size of the image of the scan line that is focused onto the surface of the photosensor. For example, many optical systems have a lens reduction ratio of about 8:1, which reduces the size of the image of the scan line by a factor of about 8.
While various types of photosensor devices may be used in optical scanners, a commonly used sensor is the charge coupled device or CCD. As is well-known, a CCD may comprise a large number of individual cells or "pixels," each of which collects or builds-up an electrical charge in response to exposure to light. Since the size of the accumulated electrical charge in any given cell or pixel is related to the intensity and duration of the light exposure, a CCD may be used to detect light and dark spots on an image focused thereon. In a typical scanner application, the charge built up in each of the CCD cells or pixels is measured and then discharged at regular intervals known as exposure times or sampling intervals, which may be about 5 milliseconds or so for a typical scanner. Since the charges (i.e., image data) are simultaneously collected in the CCD cells during the exposure time, the CCD also includes an analog shift register to convert the simultaneous or parallel data from the CCD cells into a sequential or serial data stream. A typical analog shift register comprises a plurality of "charge transfer buckets" each of which is connected to an individual cell. At the end of the exposure time, the charges collected by each of the CCD cells are simultaneously transferred to the charge transfer buckets, thus preparing the CCD cells for the next exposure sequence. The charge in each bucket is then transferred from bucket to bucket out of the shift register in a sequential or "bucket brigade" fashion during the time the CCD cells are being exposed to the next scan line. The sequentially arranged charges from the CCD cells may then be converted, one-by-one, into a digital signal by a suitable analog-to-digital converter.
In most optical scanner applications, each of the individual pixels in the CCD are arranged end-to-end, thus forming a linear array. Each pixel in the CCD array thus corresponds to a related pixel portion of the illuminated scan line. The individual pixels in the linear photosensor array are generally aligned in the "cross" direction, i.e., perpendicular to the direction of movement of the illuminated scan line across the object (also known as the "scan direction"). Each pixel of the linear photosensor array thus has a length measured in the cross direction and a width measured in the scan direction. In most CCD arrays the length and width of the pixels are equal, typically being about 8 microns or so in each dimension.
The resolution in the cross direction is a function of the number of individual cells in the CCD. For example, a commonly used CCD photosensor array contains a sufficient number of individual cells or pixels to allow a resolution in the cross direction of about 600 pixels, or dots, per inch (dpi), which is referred to herein as the "native resolution in the cross direction."
The resolution in the scan direction is inversely related to the product of the scan line sweep rate and the CCD exposure time (i.e., the sampling interval). Therefore, the resolution in the scan direction may be increased by decreasing the scan line sweep rate, the CCD exposure time, or both. Conversely, the resolution in the scan direction may be decreased by increasing the scan line sweep rate, the CCD exposure time, or both. The "minimum resolution in the scan direction" for a given exposure time is that resolution achieved when scanning at the maximum scan line sweep rate at that exposure time. For example, a maximum scan line sweep rate of about 3.33 inches per second and a maximum exposure time of about 5 milliseconds will result in a minimum resolution in the scan direction of about 60 dpi.
The resolution in the cross direction may be decreased below the native resolution in the cross direction by using any one of a number of pixel dropping algorithms to ignore, or drop, data from certain cells in the CCD. For example, the resolution in the cross direction in a CCD having a native resolution of 600 dpi may be decreased to 300 dpi by ignoring or dropping data from every other pixel. Most commonly used pixel dropping techniques ignore or drop the pixel data after it has been converted into a digital signal by the analog-to-digital converter. It is also possible to increase the resolution in the cross direction over the native resolution in the cross direction by using various data interpolation techniques to increase the effective resolution in the cross direction. For example, some data interpolation techniques can be used to increase the effective resolution in the cross direction to 1200 dpi or more with a CCD having a native resolution in the cross direction of only 600 dpi.
As mentioned above, the resolution in the scan direction is a function of the scan line sweep rate as well as the CCD exposure time. Therefore, the resolution in the scan direction can be varied by changing the scan line sweep rate, the CCD exposure time, or both. It should be noted that resolution in the scan direction corresponding to a given maximum scan line sweep rate and CCD exposure time is fixed and represents the minimum resolution in the scan direction for that exposure time. However, the resolution in the scan direction may be further reduced by ignoring or dropping whole lines of data. Such line dropping techniques are analogous to the pixel dropping techniques described above.
One problem associated with scanners that drop pixels to decrease the resolution in the cross direction, or drop lines to decrease the resolution in the scan direction, or both, is that the pixel and line dropping processes tend to introduce various artifacts and distortions into the image data, such as alising and moire patterns.
Another problem associated with pixel and line dropping processes is that the pixel and line dropping functions are usually performed after the charge data from the individual CCD cells have been converted into digital form. Consequently, the maximum sampling rate, thus scanning speed, is limited to the data conversion rate of the analog to digital (A/D) converter. Since most scanners operate at the maximum effective sampling rate of the A/D converter, the scanning rate when scanning at reduced resolution is essentially the same as when scanning at maximum resolution. Put in other words, selecting a decreased resolution will not usually result in an increased scan rate.
Even if faster analog to digital converters are used, there is a limit to the maximum scan rate that can be achieved. For example, the provision of a faster analog-to-digital converter will allow faster scan rates at a given resolution only if the exposure time (i.e., sampling interval) is decreased. However, since the amount of charge produced by a given CCD cell is proportional to the exposure time, shorter exposure times will result in proportionally lower signal levels. Assuming constant system noise, such lower signal levels will yield a lower signal to noise ratio, which noise usually appears in the image data as "snow."
Therefore, there remains a need for an image scanner that can scan at a wide range of resolutions but without the image degradation problems, such as alising, and the generation of moire patterns, that are typically associated with the line dropping processes typically used in currently available scanners. Ideally, the selection of a decreased scanning resolution should also result in a corresponding increase in scanning speed, but without the need to resort to expensive, high-speed analog-to-digital converters and without reducing the signal-to-noise ratio of the resulting image data signal.